1. Field of the Invention
The present invention relates to a hot bulge forming apparatus and more particularly to a hot bulge forming die apparatus for forming a pre-heated tubular workpiece.
2. Related Art
Conventionally, there has been known a hot bulge forming process in which high-pressure air is supplied into a tubular workpiece disposed between dies so as to form the tubular workpiece into a shape of a cavity defined between the dies.
Specifically, in this hot bulge forming process, for example, a tubular workpiece is preheated, and the tubular workpiece so heated is disposed between a pair of dies. Next, the dies are clamped together while the workpiece is restrained at both lengthwise ends thereof, and high-pressure air is supplied into the tubular workpiece so that the workpiece is pressed against cavity surfaces of the dies. Thereafter, this state is maintained for a certain length of time to cool the workpiece by the dies, whereafter the dies are opened to remove the workpiece so formed from the dies (for example, refer to US2005/0029714).
Here, a projection is formed on a circumferential portion of the cavity surface of one of the dies, while a hole is formed in a circumferential edge portion of the cavity surface of the other die so that the projection fits in the hole with no gap left between the projection and the hole. Then, when clamping the dies together, the circumferential edge portions of the dies are joined together so that the projection on the one die fits in the hole in the other die, whereby the circumferential edge portions of the pair of dies are restrained by each other.
Incidentally, when forming workpieces one after another by the hot bulge forming dies, there has been caused a problem that dimensions of workpieces gradually increase until the number of times of forming reaches a certain number of times of forming.
Namely, in the hot bulge forming process, as a result of cooling a formed workpiece by the dies, the temperature of the dies before another forming is started remains much lower than that of a workpiece.
When a workpiece is introduced into the dies to start forming from that state, the dies absorb heat of the workpiece and expand thermally, whereby the dies warp outwards. Consequently, although the circumferential edge portions of the pair of dies are restrained by each other, the circumferential edge portions are offset from each other.
Consequently, since the quantity of heat that the dies absorb from a workpiece every time forming is performed is increased, the degree of deformation of the dies due to warping is gradually increased, and the degree of offset between the circumferential edge portions is also increased.
Thereafter, when the quantity of heat that the dies absorb from a workpiece and the quantity of heat that is emitted from the dies come to be in balance after the forming has been repeated a certain number of times, a difference in temperature between an inside and an outside of the dies becomes constant, and the degree of deformation of the dies becomes constant, whereby the shapes of the dies become stable.
Consequently, after the number of times of forming has reached a certain number of times and the shapes of the dies have been stabilized, the dimensions of the formed products become almost constant. However, the dimensions of the formed products gradually increase until the shapes of the dies become stable, and hence, the dimensions of the formed products do not become constant.
With a view to solving the problem, in the aforesaid hot bulge forming process, there are proposed two approaches.
A first approach is an approach in which the products formed before the shapes of dies become stable are disposed of as defectives, and only the products formed after the shapes of the dies become stable are adopted as proper products. In this case, the dies are designed in consideration of deformation of the dies due to thermal expansion thereof in advance.
With this first approach, however, due to the products formed immediately after the start of forming being disposed of, the production costs are increased.
A second approach is an approach in which thicknesses of dies are increased so as to increase the rigidity thereof to thereby suppress the deformation of the dies due to thermal expansion thereof. With this approach, since the deformation of the dies can be suppressed in an ensured fashion, irrespective of the number of times of forming, the dimensions of the formed products can be made constant.
With this second approach, the dies and peripheral equipment are made large in size, resulting in high production costs.